The present invention relates to a method of manufacture of an optical data storage disc in which there is at least one layer of information and a further layer, spaced from said first layer or layers, which may also be an information layer or be provided with some other micro-relief pattern, as examples a micro-relief pattern to present a holographic image, other optically variable devices and digital information (pits and lands).
Optical data storage discs have been known for some time, one example being known commercially as a compact disc, such a disc usually comprising a layer of plastics material, on one side of which is formed a micro-relief pattern in the form of indentations which comprise an encoded signal. The side of the disc provided with the micro-relief pattern is then metallised to provide a reflective layer and coated with a lacquer to protect the metallised layer. The information layer is then read by a laser through the plastics layer.
Another application for this invention is a form of optical disc known as DVD which can have more than one information layer. A DVD disc has several formats but always comprises two substrates each formed with at least one information layer. The two substrates are then secured to each other.
The side of the disc covered with lacquer is often provided with printed information indicating the contents of the disc, e.g. the name of the orchestra or singer as the case may be. It is also known to provide some artistic design.
European Patent EP 0608358B1 describes an optical data storage disc in which a micro-relief pattern is embossed in the lacquer forming the protective layer for the metallised surface, the micro-relief pattern defining a holographic image.
Compact discs are normally formed from polycarbonate material which is injected into an injection moulding machine in which a stamper is present, the stamper forming the pattern of indentations in the subsequently formed disc to provide the encoded signal. Other techniques use a compression moulding technique. In both cases the substrate formed has the majority of both surfaces flat, for example within 2 microns.
As aforementioned, the majority of both the surfaces of the substrate are flat, however, the manner in which the disc is formed leads to an increase in substrate thickness hereafter referred to as a peripheral ridge. The precise dimension of the peripheral ridge varies slightly, depending upon the mould used. However, typically, the ridge may commence approximately 5 mm from the periphery of the substrate, the height of the ridge gradually increasing towards the periphery to an amount typically 50 microns above the average substrate thickness elsewhere.
Such a ramp-like ridge may be formed by both sides of the substrate.
The extension above the otherwise flat surface of the polycarbonate substrate is therefore out of all proportion compared to the general surface flatness of 1 or 2 microns and the height of this peripheral ridge is further accentuated when the surface of the disc is covered with a layer of protective lacquer which can be in a wide range of thickness (1 to 100 um).
In practical optical disc production, the lacquer layer may typically be in the region of 4-12 microns thick over the majority of the surface. However, the thickness of the lacquer may be 2 or 3 times greater in the region of the peripheral ridge. The height of the peripheral ridge above the lacquered surface of the disc is therefore considerable.
It has been found that if one attempts to emboss a holographic micro-relief pattern in the protective lacquer, the presence of the peripheral ridge prevents a proper embossing of the disc and at best, leaves a patchy holographic image.
It is an object of the present invention to provide a method of providing a micro-relief pattern on an optical data storage disc.